SKIN, SCALES, AND SPINES 85 



in patches like a snake, a new and brightly coloured skin 

 developing below the old one. 



The scales of a fish are products of the activity of the skin, 

 and owe their existence to the presence of lime salts in the 

 tissues of the body, which become deposited in the dermis. 

 The form of the scales, spines, or other related structures varies 

 considerably in the different groups of fishes, and provides an 

 important character for their classification. 



In the Cyclostomes scales are altogether wanting, but a study 

 of their fossil ancestors suggests that this is a feature of de- 



FlG. 36. — DERMAL DENTICLES. 



a. Isolated denticles of the Spotted Dog-fish (Scyliorhimis caniculus), greatly 

 enlarged ; b. Diagrammatic cross-section of a denticle of a Selachian, showing 

 the enamel covering and the central pulp cavity, greatly enlarged ; c. Portion of 

 skin with dermal denticles of the Bramble Shark {Echinorhinus spinosus), X | ; 

 d. "Buckler" of Thornback Ray {Rata clavata), lateral and dorsal view, xf ; 

 e. Tail-spine of Sting Ray {Trygon sp.), X \. 



generation rather than a primitive character. The dermal 

 denticles of Sharks and Rays, sometimes known as odontoids 

 or placoid scales, almost certainly represent the most primitive 

 type of scale, and may conveniently be described first. The 

 surface of a shark's body is generally prickly to the touch, due 

 to the presence of innumerable tooth-like structures arranged 

 in regular oblique rows, covering the whole of the head, body, 

 and part of the fins. Each of these denticles consists of two 

 portions, a bone-like base which is embedded in the skin and 

 therefore invisible during life, and a superficial enamel-covered 

 spine projecting freely outwards and backwards (Fig. 36a, b). 

 Such denticles provide the familiar "shagreen," and the 



